I've had my Gear S for five days now. Samsung sent me the wrong SIM card, which was too small to fit my Gear S. Fortunately, T-Mobile gave me the correct SIM card when I was in their store. They had to mail the Gear S to my home though, because they don't have any Gear S units in their store.
However, Samsung had my Gear S registered to the wrong SIM card, which was causing some serious issues. After calling Tech Support many times (both T-Mobile and Samsung), finally one of the Tech Support discovered that my Gear S was registered with a different SIM card (the wrong one Samsung sent to me, which was too small for my device). When she fixed the issue, my Gear S has been working nearly flawlessly!